June 16, 2010 -- Vitamin D may reduce the incidence and severity of influenza and other infections of the upper respiratory tract, new research indicates.

Simple steps such as eating foods rich with vitamin D and getting more sunshine may help to reduce your chances of contracting flu and other similar illnesses, shows a study by scientists at Yale University School of Medicine and Greenwich Hospital in Connecticut.

"People in the South and West get more sun than in the North, which is good for them, because you get vitamin D from the sun," study researcher James R. Sabetta, MD, of the Yale University School of Medicine and Greenwich Hospital, Conn., tells WebMD. "It's not a panacea, but it helps."

Sabetta and his team of colleagues followed 198 healthy adults during the fall and winter of 2009-2010 to see if declining levels of vitamin D in the fall and winter could be a factor in the seasonal increased prevalence of respiratory viral infections, such as flu.

The study shows people who maintain vitamin D blood levels of 38 nanograms per milliliter or more are less likely to get viral infections such as flu than people with less in their blood.

Of 18 people who maintained that level during the study period, only three developed viral infections.

But of the 180 other participants with less vitamin D in their blood, 81(45%), did get sick with viral infections.

And those with higher levels of vitamin D also experienced a marked reduction in the number of days they were ill, Sabetta tells WebMD.

In addition to getting more sun and consuming milk and foods with vitamin D, he recommends supplements, especially for people in areas with less sunlight and for those who spend daylight hours in darker, indoor environments.

"If you have a level of 38, your risk is down 50%," he tells WebMD. "A lot of people don't have an adequate level, and 38 is a little above what you should have to be considered in the sufficient range. There are a billion people worldwide with levels below 30."

And 30 is considered "sufficient," he says.

Watching for Signs of Illness

Participants in the study had blood samples drawn monthly using a sophisticated technique to accurately measure vitamin D levels. They didn't know that vitamin D was being measured, and even investigators didn't know until the end of the study.

All participants were asked to report signs of illness, such as nasal congestion, sore throat, cough with or without fever, chills, fatigue and general malaise.

Those reporting any symptoms were seen the same day at the study site by one of the infectious disease investigators.

People in the study kept a diary of symptoms and were called every one to three days during the illness to review any signs of symptoms until they were better. The investigators recorded the duration of each symptom, the total duration of the illness, and any antimicrobials administered.

Sabetta says the findings suggest that supplementing vitamin D to achieve a blood level of 38 nanograms per milliliter or higher could result in a significant health benefit by reducing odds of contracting viral infections of the respiratory tract.

But he says more studies are needed to determine the efficacy of vitamin D supplementation in the prevention of infections, including influenza.

The researchers conclude that the lower levels of vitamin D seen during the winter in temperate climates may contribute to the prevalence of influenza in colder months.

The findings, Sabetta says, have significant implications for public health and also may explain the seasonality of certain infections, and also the higher morbidity and mortality of such illnesses in people who are predisposed to lower concentrations of vitamin D.

Sabetta says vitamin D has known effects on the immune system, and the study reinforces the association between vitamin D deficiency and susceptibility to infections of the respiratory tract.

The study is published online in the journal Plos ONE.

Vitamin D levels depend "on how big you are, your skin color, your diet and how much sun exposure you get," Sabetta tells WebMD. "Individuals should get their vitamin D levels checked. If you are gardening a lot, you probably are fine, but people in an office all day may need supplements."

SOURCES: News release, Greenwich Hospital.

Sabetta, J. Plos ONE, June 16, 2010.

James Sabetta, MD, Yale University School of Medicine, Greenwich Hospital, Greenwich, Conn.